State of the art ultraviolet (UV) nitride semiconductor light-emitting diodes (LEDs) are relatively inefficient compared to their visible counterparts. The inefficiency is due a number of factors including (1) lower internal quantum efficiencies (a lower probability that an injected carrier is converted to a UV photon by radiative recombination) and 2) poor extraction efficiency (only a small fraction of UV photons are successfully extracted from the chip rather than being lost to internal absorption in the LED structure). In order to realize high efficiency LEDS, both of these issues need to be addressed. However the potential gain from improving extraction efficiency is likely to be greater and simpler to accomplish than the gains from improving internal efficiency.
Improving light extraction in a nitride LED is difficult due to the constraints associated with forming an electrode on a p-type semiconductor. These difficulties are described in H. Hirayama, “Quartenranry InAlGaN-based high-efficiency UV LEDs”, Journal of Applied Physics, 97, 091101 (2005). Specifically, high-work-function metals (e.g., nickel, chromeAg is a low work-function material, some rare-earth metals such as palladium and ruthenium, nickel-gold composites etc.) which are known to form good ohmic contacts with semiconductors are optically absorbing in the ultraviolet portion of the spectrum. Thus nitride LEDs typically remove light from the top (p-side)
In contrast, for visible nitride LEDs, contact metallurgies with very high reflectivity have been developed. The high reflectivities along with surface-texturing have enabled extraction efficiencies approaching 80% in visible wavelength LEDs. An analogous high reflectivity contact for UV LEDs would greatly improve UV LED performance and enable UV LEDs where light is removed from the bottom.
Thus a highly reflective p-side contact for UV LEDs is needed.